Field of the Invention
Some of constitutional examples of the present invention relate to a magnetic recording medium. In particular, some of constitutional examples of the present invention relate to a magnetic recording medium used in a hard disc magnetic recording device (HDD).
Description of the Related Art
Perpendicular magnetic recording system is adopted as a technique for increasing the magnetic recording density. A perpendicular magnetic recording medium at least comprises a non-magnetic substrate and a magnetic recording layer formed of a hard-magnetic material. Optionally, the perpendicular magnetic recording medium may further comprise: a soft-magnetic under layer playing a role in concentrating the magnetic flux generated by a magnetic head onto the magnetic recording layer; an interlayer for orienting the hard-magnetic material in the magnetic recording layer in an intended direction; a protective film for protecting the surface of the magnetic recording layer; and the like.
The magnetic recording layer of the perpendicular magnetic recording medium formed of a granular magnetic material has been proposed for the purpose of obtaining favorite magnetic properties. The granular magnetic material comprises magnetic crystal grains and a non-magnetic substance segregated to surround the magnetic crystal grains. Respective magnetic crystal grains in the granular magnetic material are magnetically separated from each other with the non-magnetic substance.
For the purpose of further increasing the recording density of the perpendicular magnetic recording medium, a need for reduction in the grain diameter of the magnetic crystal grains in the magnetic layer arises in recent years. On the other hand, the reduction in the grain diameter of the magnetic crystal grains leads to a decrease in thermal stability of the recorded magnetization (also referred to as signals). Thus, the magnetic crystal grains in the magnetic layer need to be formed of materials with higher magnetocrystalline anisotropies, in order to compensate the decrease in thermal stability due to the reduction in the grain diameter of the magnetic crystal grains. As the materials having the required higher magnetocrystalline anisotropies, L10 type ordered alloys have been proposed. Typical L10 type ordered alloys include FePt, CoPt, FePd, CoPd, and the like.
Various trial to improve the magnetic properties of the magnetic recording layer containing the L10 type ordered alloy by the layer formed under the magnetic recording layer has been made. For example, Japanese Patent Laid-Open No. 2011-165232 proposes a magnetic recording medium wherein the magnetic recording layer is formed on an interlayer containing MgO as a main component and one or more of additional oxides. In this proposal, a crystal grain diameter of the crystal grains in the interlayer is reduced and one magnetic crystal grain in the magnetic recording layer is formed on one crystal grain in the interlayer (hereinafter, referred to as “one-to-one formation”), thereby making it possible to improve separation among the magnetic crystal grains, to decrease exchange coupling, and to decrease dispersion of coercive force.
International Publication No. WO 2011/021652 proposes a magnetic recording medium in which a third interlayer consisting of MgO is formed on a second interlayer comprising Cr and at least one additional element selected from the group consisting of Ti, Mo, W, V, Mn and Ru, and then the magnetic recording layer is formed on the third interlayer. This proposal is based on the fact that the crystal grain diameter of the second interlayer is reduced by adding the additional element to the second interlayer. The reduction in the crystal grain diameter of the second interlayer leads to reduction in crystal grain diameter of the third interlayer consisting of MgO, thereby allowing the “one-to-one formation” between the third interlayer and the magnetic recording layer. Then, exchange coupling among the magnetic crystal grains and media noise can be reduced by the effect of the “one-to-one formation”.
Japanese Patent Laid-Open No. 2011-146089 proposes a magnetic recording medium in which a seed layer consisting of amorphous ceramics such as SiO2, an orientation controlling layer consisting of crystalline material such as MgO and MnAl2O4, and a magnetic layer comprising an FePt alloy as a main component are stacked in this order. In this proposal, improvement in crystalline orientation of the orientation controlling layer and miniaturization of the structure of the orientation controlling layer are made possible by disposing the seed layer consisting of amorphous ceramics. Then, prevention of disturbance in the crystalline orientation of the FePt alloy and miniaturization of grain diameter of the magnetic crystal grains in the magnetic layer leads to the magnetic recording layer having a high magnetic anisotropy constant Ku, excellent magnetic properties and excellent read-write characteristics.
When improvement in recording density of the magnetic recording layer is intended, one of factors to be considered is a distance between the magnetic recording layer and a magnetic head. Generally, the shorter distance between the magnetic recording layer and the magnetic head makes it possible to improve the recording density. On the other hand, the distance between the magnetic recording layer and the magnetic head must be larger than the surface roughness of the magnetic recording medium. If the surface roughness of the magnetic recording medium increases, the read-write characteristics of the magnetic recording medium may be deteriorated by undesirable influence of the surface roughness upon flying characteristics of the magnetic head. In the case of the conventional interlayer consisting of MgO, it is necessary to increase the substrate temperature to 400° C. or higher in order to achieve the desired surface roughness. Therefore, there is a need to reduce the surface roughness of the magnetic recording medium, especially the surface roughness of the magnetic recording layer.